METHOD FOR PRODUCING Al-KILLED DEEP DRAWING COLD ROLLED STEEL PLATE BY CONTINUOUS ANNEALING

ABSTRACT

A method of producing an aluminum-killed cold rolled steel sheet for deep drawing by a continuous annealing, which comprises cold rolling a hot rolled steel plate containing not more than 0.015 percent of carbon, 0.05 to 0.50 percent of manganese, 0.13 to 0.33 percent of soluble aluminum with the balance being iron and unavoidable impurities.

United States Patent Shirnizu et a1.

[ Mar. 19, 1974 METHOD FOR PRODUCING AL-KILLED DEEP DRAWING COLD ROLLEDSTEEL PLATE BY CONTINUOUS ANNEALING Inventors: Mineo Shirnizu; HiroshiTakechi;

Kenichiro Suemune; Setsuo Nagao; Kuniteru Ohta, all of Kitakyusyu, JapanNippon Steel Corporation, Tokyo, Japan Filed: Apr. 27, 1972 Appl. No.:248,242

Assignee:

Foreign Application Priority Data Apr. 27, 1971 Japan 46-027179 U.S. C1.148/12 Int. Cl C21d 9/48 Field of Search 75/124; 148/12 [56] ReferencesCited UNITED STATES PATENTS 3,248,270 4/1966 Laidman et a1 148/123,522,110 7/1970 Shimizu et a1. 148/12 3,615,925 10/1971 Garber et a]148/12 3,666,569 5/1972 Hultgren 148/12 3,668,016 6/1972 Shimizu et a1.148/12 Primary Examiner-W. W. Stallard Attorney, Agent, orFirmWenderoth, Lind & Ponack [5 7 ABSTRACT A method of producing analuminum-killed cold rolled steel sheet for deep drawing by a continuousannealing, which comprises cold rolling a hot rolled steel platecontaining not more than 0.015 percent of carbon, 0.05 to 0.50 percentof manganese, 0.13 to 0.33 percent of soluble aluminum with the balancebeing iron and unavoidable impurities.

2 Claims, 1 Drawing Figure Box Annealing (Temp, 700C, Soaking: 4 hours)Continuous Annealing (Tempi 850C,

Soaking: 5 minutes) F value value PATENTEDHAR i 9 m4 3,798,076

Box Annealing (Temp. 700C, Soaking: 4 hours) Continuous Annealing (Temp.850C,

Soaking: 5 minutes) METHOD FOR PRODUCING AL-KILLED DEEP DRAWING COLDROLLED STEEL PLATE BY CONTINUOUS ANNEALIING BACKGROUND OF THE INVENTIONThe present invention relates to a method for producing cold rolledsteel plate or sheet having excellent deep-drawability.

Cold rolled steel sheets have been widely used in various applicationssuch as in the automobile industries, and for electric applications,office utensils, etc.

Among these applications, aluminum-killed cold rolled steel sheets arecommonly used in cases where deep-drawability is required.

The aluminum-killed cold rolled steel sheets have been conventionallyproduced without exception by recrystallization annealing with gradualheating of cold rolled steel sheet in a coil form in a box annealingfurnace. There has never been developed a continuous annealing processwhich is a short-time annealing process with an accelerated heating andcooling rate because such process will sacrify desirous properties ofaluminum-killed steel sheets.

On the other hand, the box annealing method has such defects thatproduction per unit time is small and the production cost is increasedbecause of the very low heating and cooling rate of the coil.

SUMMARY OF THE INVENTION In the course of extensive experiments inefforts for solving difficulties in the production of aluminumkilledcold rolled steel sheets having excellent deep-drawability by continuousannealing, the present inventors have discovered that when a cold rolledsteel sheet having a higher soluble aluminum content than that of theconventionally produced aluminum-killed steel sheets (0.03 0.06 percentof sol.Al), namely when the sheet contains a soluble aluminum contentbetween 0.13 percent and 0.33 percent, is continuously annealed with aheat cycle above the recrystallization temperature, an average plasticstrain ratio (T value) similar to or more than that of the conventionaluminum-killed steel sheet is obtained. The present invention is basedon the above discovery.

BRIEF DESCRIPTION OF THE DRAWING The present invention will be describedin detail with reference to the attached drawing, which shows a graphindicating 'r' values obtained when aluminum-killed steel sheets havingdifferent sol.Al contents are continuously annealed and box annealed.

DETAILED DESCRIPTION OF THE INVENTION Hot rolled steel sheets used asthe starting materials in the present invention may be produced byordinary production processes, including ingot-making and breaking downfrom molten steel prepared in electric furnaces, convertors, etc. orfrom molten steel further treated by vacuum degassing; slab-making bycontinuous casting, hot rolling and then acid pickling. Among hot rolledsteel plates produced as above, it is found that the steel compositionwhich show excellent deep-drawability by continuous annealing must bedifferent from the composition of the conventional aluminum-killed hotrolled steel plates. Thus the steel composition is: less than 0.015percent of carbon, 0.05

0.50 percent of manganese, 0.13 0.33 percent of soluble aluminum withthe balance being iron and unavoidable impurities.

The range of the above composition is determined from the productionconditions of the hot rolled steel sheet and the deep-drawability afterthe continuous annealing.

More concrete explanations on the above limitations of each element ofthe steel composition will be made hereinunder.

Carbon is effective and necessary for giving appropriate strength to thecold rolled steel, but too high a content of carbon lowersdeep-drawability due to dissolution of over-saturated carbon into thematrix during the continuous annealing where accelerated heating andcooling is done, and forms lump cementite due to the coagulation ofprecipitates taking place during the cooling step after the hot rollingin the production of hot rolled steel plates. This cementite is finelydevided by the cold rolling and locally dispersed, which causes variousdefects such as lowering of deep-drawability. Therefore, for improvementof deep-drawability it is desired that the carbon content be as low aspossible, and the upper limit of the carbon content range is set at0.015 percent with consideration to other properties and productioncost. For lowering the carbon content, decarburization annealing of hotrolled steel sheets as well as decarburization of molten steel may beconducted.

Manganese is effective to prevent hot embrittlement due to sulphur whenit is contained in an amount more than 0.05 percent. However, too high amanganese content increases strength and lowers deep-drawability.Therefore, the upper limit of manganese is set at 0.5 percent. The rangeof soluble aluminum of 0.13 to 0.33 percent has been determined from theresults of experiments on the present invention. This range has beenselected so as to assure an value similar to or more than that obtainedby box annealing conventional aluminum-killed cold rolled steel sheets,and particularly the range of 0.18 to 0.30 percent is desired. When analuminum-killed cold rolled steel sheet containing such a solublealuminum content is continuously annealed, deep-drawability is improved.This improvement may be attributed to the assumption that theacceralated heating and cooling rate as effected in continuous annealingis favourable for AlN present in the steel to form-a coagulatedstructure favourable to deep-drawing during the restoration andrecrystallization period. Soluble aluminum contents excessively out ofthis range are considered to cause problems unfavourable to thecoagulated structure or the Al precipitates, thus loweringdeep-drawability. It is desirable that unavoidable impurities such as Pand S be maintained as low as possible due to their adverse effect oflowering deep-drawability.

According to the present invention, a hot rolled steel sheet of theabove composition is cold rolled at a reduction rate of more than 30percent and subjected to recrystallization annealing in a continuousannealing furnace.

In this case, the heat cycle, namely the heating and cooling rate, andthe soaking time in the annealing treatment and so on are notspecifically limited.

The method of the present invention has a remarkable advantage in thatan aluminum-killed cold rolled steel sheet having betterdeep-drawability than that of the conventional aluminum-killed coldsteel sheet produced by box annealing can be produced in a large amountat low production cost.

This is shown in the single FIGURE which shows the effect of annealingmethod on theTvalues. The test was duplicated. The dotted lines show Fvalues obtained when the steels were subjected to continuous annealingat 850C for 5 minutes, while the solid lines show ?values obtained whenthe steels were box-annealed at 700C for 4 hours. As is clear from theFlGURE, the effect of the sol. Al content on the F value is notremarkable when the steel is box-annealed. Thus the F value becomes higharound 0.15 percent sol. Al content but lowers when so]. A1 content ismore than about 0.15 percent. On the other hand, when the steel issubjected to continuous annealing, the F value is remarkably improved asthe sol. Al content is more than the 0.1 1 percent, and around 0.13percent sol. A1 content the F value exceeds that of the box-annealedsteel, and it shows a peek around 0.22 percent sol. A1 content.

An example of the present invention will be set forth below. 1

Table 1 shows chemical compositions of hot rolled steel sheets producedfrom vacuum degassed molten steel prepared in a convertor byingot-making, breaking-down and hot rolling. The hot rolled steel plateswere cold rolled at a reduction rate of 70 percent to oband as comparedwith M steel and N steel which were produced by box annealing.

Table 2 Mechanical Properties of Steel Sheets As Annealed Yield TensileRupture T value Stress Strength Elongation (Kg/m (Kg/m A 21.2 31.5 45.21.65 Inventive B 19.6 30.1 46.0 1.79 Steel C 19.3 30.3 47.2 1.91Sheets 1) 20.1 30.) 46.1 1.141 F. 21.2 31.1 44.8 1.611

F 20.11 30.6 46.2 1.41 G 21.6 31.8 45.6 1.46 H 24.8 33.1 43.2 1.29Comparison .1 27.2 34.2 46.2 1.28 Steel K 286 34.8 43.8 1.12 Sheets BoxAnnealing (Temperature: 700C Soaking: 5 hours) Table 1 (weight Grades CSi Mn P sol. Al N A 0.005 0.01 0.09 0.010 0.008 0.150 0.0042 lnventive B0.009 0.31 0.012 0.012 0.186 0.0036 Steel C 0.006 0.01 0.16 0.011 0.0110.243 0.0052 Sheets D 0.34 0.017 0.014 0.310 0.0061 E 0.012 0.02 0.310.013 0.011 0.192 0.0055 F 0.005 0.01 0.21 0.016 0.017 0.072 0.0048 G0.009 0.18 0.015 0.014 0.106 0.0062 H 0.007 0.02 0.61 0.012 0.241 0.0051Compari- 1 0.023 0.01 0.31 0.017 0.016 0.210 0.0056 son J 0.040 0.340.014 0.011 0.216 0.0048 Steel K 0.058 0.29 0.012 0.014 0.056 0.0055Sheets L 0.026 0.35 0.014 0.013 tr 0.024

(Rimmed Steel) M 0.049 0.31 0.016 0.016 0.063 0.0041 N 0.033 0.33 0.0170.015 tr 0.0021

(Rimmed Steel) What is claimed is: l. A method of producing analuminum-killed cold rolled steel sheet for deep drawing, said methodcomprising: cold rolling a hot rolled steel plate containing not morethan 0.015 percent of carbon, 0.05 to 0.50 percent of manganese, 0.13 to0.33 percent of soluble aluminum, with the balance being iron andunavoidable impurities, and continuously annealing the thus cold rolledsteel sheet.

2. A method according to claim 1, wherein said hot rolled steel platecontains 0.18 to 0.30 percent of soluble aluminum.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION 5 Patent No.3,798,076 7 Dated March 19, 1974 Inventor(s) Mineo Shimizu, HiroshiTakechi, Kenichiro Suemune,

Setsuo Nagao, and Kuniteru Ohta. It is certified that error appears inthe above-identified patent and that said Letters Patent are herebycorrected as shown below:

In the single Figure of the drawings, change the "sol Al(%)" legendsfrom ".01", ".02" and ".03" to .l .2 and .3 respectively.

Signed and sealed this 30th day of July 197 SEAL Attest:

MCCOY M. GIBSON, JR. Attesting Officer C. MARSHALL DANN CommissionerofPatents FdRM PC4050 'USCOEAM-DC 60376-P69 LS. GOVERNMENT PRINTING OFFICEIQ, -3-33

2. A method according to claim 1, wherein said hot rolled steel platecontains 0.18 to 0.30 percent of soluble aluminum.